Add rail-fence-cipher exercise

config.json

@@ -540,6 +540,14 @@
         "prerequisites": [],
         "difficulty": 5
       },
+      {
+        "slug": "rail-fence-cipher",
+        "name": "Rail Fence Cipher",
+        "uuid": "252f263c-9a4a-4c18-8c23-0a1fcc94b36a",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 6
+      },
       {
         "slug": "connect",
         "name": "Connect",

exercises/practice/rail-fence-cipher/.docs/instructions.md

@@ -0,0 +1,57 @@
+# Instructions
+
+Implement encoding and decoding for the rail fence cipher.
+
+The Rail Fence cipher is a form of transposition cipher that gets its name from the way in which it's encoded.
+It was already used by the ancient Greeks.
+
+In the Rail Fence cipher, the message is written downwards on successive "rails" of an imaginary fence, then moving up when we get to the bottom (like a zig-zag).
+Finally the message is then read off in rows.
+
+For example, using three "rails" and the message "WE ARE DISCOVERED FLEE AT ONCE", the cipherer writes out:
+
+```text
+W . . . E . . . C . . . R . . . L . . . T . . . E
+. E . R . D . S . O . E . E . F . E . A . O . C .
+. . A . . . I . . . V . . . D . . . E . . . N . .
+```
+
+Then reads off:
+
+```text
+WECRLTEERDSOEEFEAOCAIVDEN
+```
+
+To decrypt a message you take the zig-zag shape and fill the ciphertext along the rows.
+
+```text
+? . . . ? . . . ? . . . ? . . . ? . . . ? . . . ?
+. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
+. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
+```
+
+The first row has seven spots that can be filled with "WECRLTE".
+
+```text
+W . . . E . . . C . . . R . . . L . . . T . . . E
+. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
+. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
+```
+
+Now the 2nd row takes "ERDSOEEFEAOC".
+
+```text
+W . . . E . . . C . . . R . . . L . . . T . . . E
+. E . R . D . S . O . E . E . F . E . A . O . C .
+. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
+```
+
+Leaving "AIVDEN" for the last row.
+
+```text
+W . . . E . . . C . . . R . . . L . . . T . . . E
+. E . R . D . S . O . E . E . F . E . A . O . C .
+. . A . . . I . . . V . . . D . . . E . . . N . .
+```
+
+If you now read along the zig-zag shape you can read the original message.

exercises/practice/rail-fence-cipher/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "RailFenceCipher.pas"
+    ],
+    "test": [
+      "TestCases.pas"
+    ],
+    "example": [
+      ".meta/example.pas"
+    ]
+  },
+  "blurb": "Implement encoding and decoding for the rail fence cipher.",
+  "source": "Wikipedia",
+  "source_url": "https://en.wikipedia.org/wiki/Transposition_cipher#Rail_Fence_cipher"
+}

exercises/practice/rail-fence-cipher/.meta/example.pas

@@ -0,0 +1,75 @@
+unit RailFenceCipher;
+
+{$mode ObjFPC}{$H+}
+
+interface
+
+function encode(const msg : String; const rails : Smallint) : String;
+function decode(const msg : String; const rails : Smallint) : String;
+
+implementation
+
+uses SysUtils;
+
+function cipher(const msg : String; const rails : Smallint; decipher: Boolean) : String;
+var
+  table : Array of Integer = ();
+  rail : Integer;
+  direction : Integer;
+  acc : Integer;
+  entry : Integer;
+  index : Integer;
+begin
+  SetLength(table, rails);
+  for rail := 0 to rails - 1 do
+    table[rail] := 0;
+
+  rail := 0;
+  direction := 1;
+  for index := 1 to Length(msg) do
+    begin
+      entry := table[rail] + 1;
+      table[rail] := entry;
+      rail := rail + direction;
+      if (rail = 0) or (rail + 1 = rails) then
+        direction := -direction;
+    end;
+
+  acc := 0;
+  for rail := 0 to rails - 1 do
+    begin
+      entry := table[rail];
+      table[rail] := acc;
+      acc := acc + entry;
+    end;
+
+  result := '';
+  SetLength(result, Length(msg));
+
+  rail := 0;
+  direction := 1;
+  for index := 1 to Length(msg) do
+    begin
+      entry := table[rail] + 1;
+      if decipher then
+        result[index] := msg[entry]
+      else
+        result[entry] := msg[index];
+      table[rail] := entry;
+      rail := rail + direction;
+      if (rail = 0) or (rail + 1 = rails) then
+        direction := -direction;
+    end;
+end;
+
+function encode(const msg : String; const rails : Smallint) : String;
+begin
+  result := cipher(msg, rails, false);
+end;
+
+function decode(const msg : String; const rails : Smallint) : String;
+begin
+  result := cipher(msg, rails, true);
+end;
+
+end.

exercises/practice/rail-fence-cipher/.meta/tests.toml

@@ -0,0 +1,28 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[46dc5c50-5538-401d-93a5-41102680d068]
+description = "encode -> encode with two rails"
+
+[25691697-fbd8-4278-8c38-b84068b7bc29]
+description = "encode -> encode with three rails"
+
+[384f0fea-1442-4f1a-a7c4-5cbc2044002c]
+description = "encode -> encode with ending in the middle"
+
+[cd525b17-ec34-45ef-8f0e-4f27c24a7127]
+description = "decode -> decode with three rails"
+
+[dd7b4a98-1a52-4e5c-9499-cbb117833507]
+description = "decode -> decode with five rails"
+
+[93e1ecf4-fac9-45d9-9cd2-591f47d3b8d3]
+description = "decode -> decode with six rails"

exercises/practice/rail-fence-cipher/Makefile

@@ -0,0 +1,15 @@
+SHELL       = /bin/bash
+MAKEFLAGS  += --no-print-directory
+DESTDIR     = build
+EXECUTABLE  = $(DESTDIR)/test
+COMMAND     = fpc -l- -v0 -g -gl -Sa -Cr -Sehnw -Fu./lib test.pas -FE"./$(DESTDIR)"
+
+.ONESHELL:
+
+test:
+	@mkdir -p "./$(DESTDIR)"
+	@cp -r ./lib "./$(DESTDIR)"
+	@$(COMMAND) && ./$(EXECUTABLE) $(test)
+
+clean:
+	@rm -fr "./$(DESTDIR)"

exercises/practice/rail-fence-cipher/RailFenceCipher.pas

@@ -0,0 +1,28 @@
+unit RailFenceCipher;
+
+{$mode ObjFPC}{$H+}
+
+interface
+
+function encode(const msg : String; const rails : Smallint) : String;
+function decode(const msg : String; const rails : Smallint) : String;
+
+implementation
+
+uses SysUtils;
+
+function encode(const msg : String; const rails : Smallint) : String;
+begin
+
+  raise ENotImplemented.Create('Please implement your solution.'); result := Copy(msg, 0, rails);
+
+end;
+
+function decode(const msg : String; const rails : Smallint) : String;
+begin
+
+  raise ENotImplemented.Create('Please implement your solution.'); result := Copy(msg, 0, rails);
+
+end;
+
+end.

exercises/practice/rail-fence-cipher/TestCases.pas

@@ -0,0 +1,81 @@
+unit TestCases;
+
+{$mode ObjFPC}{$H+}
+
+interface
+
+uses Classes, SysUtils, FPCUnit, TestRegistry, FPCUnitTestUtils;
+
+type
+  RailFenceCipherTest = class(TTestCase)
+  published
+    procedure encode_with_two_rails;
+    procedure encode_with_three_rails;
+    procedure encode_with_ending_in_the_middle;
+    procedure decode_with_three_rails;
+    procedure decode_with_five_rails;
+    procedure decode_with_six_rails;
+  end;
+
+implementation
+
+uses RailFenceCipher;
+
+// 46dc5c50-5538-401d-93a5-41102680d068
+procedure RailFenceCipherTest.encode_with_two_rails;
+const
+  phrase : String = 'XOXOXOXOXOXOXOXOXO';
+  expect : String = 'XXXXXXXXXOOOOOOOOO';
+begin
+  TapAssertTrue(Self, 'encode with two rails', expect, RailFenceCipher.encode(phrase, 2));
+end;
+
+// 25691697-fbd8-4278-8c38-b84068b7bc29
+procedure RailFenceCipherTest.encode_with_three_rails;
+const
+  phrase : String = 'WEAREDISCOVEREDFLEEATONCE';
+  expect : String = 'WECRLTEERDSOEEFEAOCAIVDEN';
+begin
+  TapAssertTrue(Self, 'encode with three rails', expect, RailFenceCipher.encode(phrase, 3));
+end;
+
+// 384f0fea-1442-4f1a-a7c4-5cbc2044002c
+procedure RailFenceCipherTest.encode_with_ending_in_the_middle;
+const
+  phrase : String = 'EXERCISES';
+  expect : String = 'ESXIEECSR';
+begin
+  TapAssertTrue(Self, 'encode with ending in the middle', expect, RailFenceCipher.encode(phrase, 4));
+end;
+
+// cd525b17-ec34-45ef-8f0e-4f27c24a7127
+procedure RailFenceCipherTest.decode_with_three_rails;
+const
+  phrase : String = 'TEITELHDVLSNHDTISEIIEA';
+  expect : String = 'THEDEVILISINTHEDETAILS';
+begin
+  TapAssertTrue(Self, 'decode with three rails', expect, RailFenceCipher.decode(phrase, 3));
+end;
+
+// dd7b4a98-1a52-4e5c-9499-cbb117833507
+procedure RailFenceCipherTest.decode_with_five_rails;
+const
+  phrase : String = 'EIEXMSMESAORIWSCE';
+  expect : String = 'EXERCISMISAWESOME';
+begin
+  TapAssertTrue(Self, 'decode with five rails', expect, RailFenceCipher.decode(phrase, 5));
+end;
+
+// 93e1ecf4-fac9-45d9-9cd2-591f47d3b8d3
+procedure RailFenceCipherTest.decode_with_six_rails;
+const
+  phrase : String = '133714114238148966225439541018335470986172518171757571896261';
+  expect : String = '112358132134558914423337761098715972584418167651094617711286';
+begin
+  TapAssertTrue(Self, 'decode with six rails', expect, RailFenceCipher.decode(phrase, 6));
+end;
+
+initialization
+RegisterTest(RailFenceCipherTest);
+
+end.